Signals transmitted by communication systems can be classified into two main types: analog signals and digital signals. In certain cases, signals that can be processed by a communication transmitter and a communication receiver are digital signals, but a transmission medium can only transmit analog signals, and accordingly, digital/analog and analog/digital conversion is required in signal transmission. In data transmission, the communication transmitter converts a digital signal into a corresponding analog signal first, which is referred to as “modulation”. When the modulated analog signal is transmitted to the communication receiver, the analog signal further needs to be sampled, quantized, and encoded, so as to be restored to the digital signal, which is referred to as “demodulation”.
Sampling is an important process for signal demodulation. A sampling result may be measured by sampling rate, which defines the number of samples extracted per second from a continuous signal to form a discrete signal. The higher the sampling rate, the higher the Signal to Noise Ratio (SNR) of the sampled signal, and the higher the fidelity of the analog signal being restored to the digital signal. Therefore, in practical use, a generally used sampling rate is a symbol rate of multirate. The symbol rate represents a rate for transmitting a basic symbol of information, and the symbol is an expression of the information.
As the complexity is high when the sampled signal is demodulated at a symbol rate of multirate, down sampling of the signal is required, so as to enable the signal to be demodulated at a symbol rate of low data rate, in which the low data rate may be a single data rate or a double data rate.
In down sampling at a symbol rate of multirate, each symbol has a sampling sequence, and each sampling sequence includes multiple sampling points. Taking a quad data rate as an example, as shown in FIG. 1, the analog signal includes four symbols, and as shown in FIG. 2, each symbol is corresponding to a sampling sequence formed of four sampling points, in which the sampling sequences are {0, 1, 2, 3}, {4, 5, 6, 7}, {8, 9, 10, 11}, and {12, 13, 14, 15} respectively.
Down sampling is to extract one sampling point or one set of sampling points from a sampling sequence corresponding to each symbol, in which the set of sampling points include two or more sampling points. As shown in FIG. 3, when one sampling point needs to be extracted from each sampling sequence, an existing down sampling method is to extract a sampling point at a fixed position of each sampling sequence, for example, extract a sampling point at a first position, and then the sampling point corresponding to each symbol after down sampling is respectively {0}, {4}, {8}, and {12}, which are down sampling points.
The sampling sequences have different sampling points which have different energy, and the larger the energy, the more beneficial to the subsequent signal processing. However, the existing down sampling method is to extract the down sampling point at a fixed position of each sampling sequence, so it cannot be ensured that the energy of the down sampling point extracted in down sampling is the largest.